This invention relates to a magnetic memory and more particularly to a non-volatile, high-density, solid magnetic memory using superconductor.
The solid magnetic memory has high reliability due to a usage of no mechanical driving parts. The solid magnetic memory may be classified largely into two types: random access type and serial access type. The core memory is a typical one of the former type while the bubble memory is another typical one of the latter. In increasing the storage density, the random access type has a limit on the reduction in the cell size because a sensor is required for each bit. On the other hand, the serial access type can increase its storage density relatively easily but an increased access time accompanied by the increased storage density poses a serious problem. In storage devices such as bubble memory in which bubbles carrying information are moved, there is a drawback that the stability of information deteriorates as information is carried on the moving bubbles.
Considering these drawbacks and merits, it is desirable to achieve an enhanced density random access memory as the non-volatile solid magnetic memory, if the sensor problem is solved.
First, we will explain the basic operation of the random access memory using magnetic material by taking an example of the core memory which is representative of the random access memory. As the core material, a magnetic material is used whose magnetization curve has a square hysteresis as shown in FIG. 1. At a bias field intensity of Hy=0, the material has a stable residual magnetization in the positive direction at point A, and in the negative direction at point B. This means the material has a bistable characteristic and the two magnetization directions can be assigned to "1" and "0", respectively, of binary number allowing the cores to be used as a storage device.
In actual devices, as shown in FIG. 2, ring cores 10 are formed of this magnetic material and three conductor wires are passed through each of the cores, with two of them arranged vertically and horizontally (along Y-axis and X-axis). The core 10 is placed at the intersection of the vertical and horizontal wires. These cores 10 are arranged in matrix, as shown in FIG. 2, to form a memory device. The principle of storing information is shown in FIG. 3. The direction 14 of magnetization of the ring core 10 is either set clockwise as shown in FIG. 3A, or counterclockwise as shown in FIG. 3B. Suppose the ring core at the initial state is magnetized, say, clockwise (negative). At a specified address, the magnetization of the core is reversed to the counterclockwise direction (positive). To achieve this, a pulse current is applied to the Y-wire and X-wire threading through the core to impose a magnetic field on the core.
The magnitude of the pulse current applied to the X-wire and Y-wire is so set that, when the magnetic field is produced by the pulse current of only the X-wire or Y-wire, the produced field will not be sufficient to reverse the magnetization direction from negative to positive. In other words, only when pulse magnetic fields produced by Y.sub.0 -wire and X.sub.0 -wire are both imposed on a core at the intersection of these two wires simultaneously, reverse magnetization occurs in the core. This is how a signal "1" is written into a desired address on the matrix.
To read the written information, pulses are applied to both X.sub.0 -wire and Y.sub.0 -wire simultaneously to reverse the magnetization of the selected core and a check is made whether a signal is output on a read wire 13. If a signal is produced, it is found that the address to which the pulses were sent contained information "1". However, this method performs a so-called destructive reading, i.e., reading information causes the contents of the core selected to become "0". If it is desired that the previous contents be retained, the core that was read from must be rewritten into with the same data. Besides, the core memory has another drawback that the bit cell size is large, making it difficult to increase the storage density.
A magnetic storage device is available which applies the above principle to a magnetic film to achieve higher storage density. The magnetic film may be formed of a soft magnetic material with a magnetic distortion constant of .lambda.=0, such as 19% Fe-81% Ni alloy, which is deposited in circle on a substrate as shown in FIG. 4. The thickness of the film is about 1000 .ANG.. During the vapor deposition process, a magnetic field is applied to give the film a uniaxial anisotropy in its plane such that the easy magnetization axis is in the direction of Y-axis.
In reversing the magnetization of the film, a field Hy acting in parallel with the easy axis but in a direction opposite to the existing magnetization direction is produced by the X.sub.0 -wire, and at the same time a field H.sub.X perpendicular to the field H.sub.Y is produced by the Y.sub.0 -wire to cause magnetization reversal through the rotation of a magnetic moment. This makes use of the rapid rotation of a magnetic moment in reversing the magnetization in a very short time of the order of 10 ns. On the other hand, for the magnetic patterns 15 to which only H.sub.Y is applied, the reverse magnetization through the magnetic moment rotation, though started, takes time and cannot occur in a short time during which the H.sub.Y is imposed. That is, reverse magnetization occurs only when both two fields from X-axis wire and Y-axis wire are applied to the selected magnetic pattern 15' simultaneously. The fields H.sub.X and H.sub.Y are generated by applying a current to vertical and horizontal ribbons placed adjacent to the deposited film.
However, this device has drawbacks that the reverse magnetization of the cell gradually relaxes to the original direction, rendering the stored information unstable and that as the magnetic film patterns are reduced in size, the sense output becomes small, making the reading of information difficult.